SAN FRANCISCO — Two unusual sea lion attacks in a San Francisco Bay cove led authorities to close the popular area to swimmers Friday as officials try to determine the reason for the aggressive behavior.

A sea lion bit a man in the groin area Friday as he swam in the waters off San Francisco Maritime National Historic Park, park spokesman Lynn Cullivan said. It was “a very serious bite,” and the man was taken to a hospital, he said.

The cove hosts swimming and rowing clubs and is a favorite spot for dedicated swimmers. It’s usually a transit area for sea lions heading to Pier 39, where they congregate, Cullivan said.

The area just off Ghirardelli Square, a popular square with shops and restaurants, will be closed to swimmers until Monday, he said.

Another swimmer was seriously injured Thursday after a sea lion bit him on the arm. Officers applied a tourniquet and he was transported to a hospital, where he had a least two surgeries.

Christian Einfeldt believes the animal trailed him as he swam from the beach to the mouth of the cove. When he turned to swim back, the massive sea lion was right there, he told San Francisco television station KGO.

“I did feel threatened. I did have warning. But I was a quarter-mile from shore. There was nothing I could do,” Einfeldt said between surgeries.

When the sea lion got near Einfeldt, he splashed water on it, but the animal did not go away, said Matthew Reiter with the San Francisco Police Department’s Marine Unit.

“When it didn’t work, he yelled at it and then the sea lion came up and bit him on the arm,” Reiter said.

He started steadily bleeding. Knowing he could not swim back, Einfeldt said he flagged down a sailboat.

“I was relatively calm because I wasn’t dead,” he told the TV station.

A man on the sailboat pulled him aboard and called police.

The aggressive behavior is so unusual for sea lions that officials think the same animal attacked both swimmers, Cullivan said.

“It could just be a sea lion being territorial, or it could be an injured or sick mammal,” he said.

Cullivan said the swimmers were closer to the bay than to the shore when they were attacked.

“People think the bay is their backyard, but it really is the beginning of the wilderness,” he said. “There is wilderness out there.”